Borohydride fuel compositions and methods

ABSTRACT

Solid self-stabilized borohydride fuel compositions, fuel cartridges, related methods of preparation and hydrogen generation are provided. The fuel compositions comprise mixtures of at least one borohydride salt with a cation selected from the group consisting of alkali metal cations, alkaline earth metal cations, aluminum cation, zinc cation, and ammonium cation, preferably sodium borohydride, and at least one hydroxide salt with a cation selected from the group consisting of alkali metal cations, alkaline earth metal cations, aluminum cation, zinc cation, and ammonium cation, preferably sodium hydroxide.

FIELD OF THE INVENTION

The present invention relates to borohydride fuel compositions, which are particularly useful in the generation of hydrogen, and to related methods of fuel manufacture and hydrogen generation.

BACKGROUND OF THE INVENTION

Although hydrogen gas is the fuel of choice for fuel cells, its use is complicated by difficulties in storing the gas. Various hydrogen carriers, including hydrocarbons, metal hydrides, and chemical hydrides are being considered as hydrogen storage and supply systems. In each case, specific systems need to be developed to release the hydrogen from its carrier, either by reformation as in the case of hydrocarbons, by desorption from metal hydrides, or by catalyzed hydrolysis from metal hydrides and water.

One of the more promising systems for hydrogen storage and generation utilizes borohydride salts as the storage media. Addition of water to borohydride salts produces hydrogen according to the reaction shown in Equation (1) below. The rate of reaction varies for different borohydrides and, for some, the use of an acidic or metal catalyst to promote the reaction is preferred. MBH₄+2H₂O→MBO₂+4H₂+heat  (1)

Sodium borohydride (NaBH4) is of particular interest because it can be dissolved in alkaline water solutions with virtually no reaction, in which case the stabilized alkaline solution of sodium borohydride is referred to as fuel. The high pH stabilizes the solution so that no rapid hydrogen generation occurs until the fuel solution contacts a catalyst. Control of this contact allows the production of hydrogen on an as-needed basis.

Typical fuel solutions comprise about 10% to 35% by weight sodium borohydride and about 0.01 to 5% by weight sodium hydroxide as a stabilizer. These aqueous borohydride fuel solutions are non-volatile and will not burn and, thus, are easily handled and transported both in bulk and within hydrogen generators. The liquid fuel is stable at temperatures below 40° C., which is sufficient for those applications which consume fuel on an ongoing manner. However, hydrogen can evolve as the temperature increases, and the fuel solution has been shown to degrade on extended storage. This is problematic in certain applications, such as in standby power generators where the fuel is stored for a period of time without hydrogen generation or consumption. In such cases, the fuel is typically required to be available at or near full strength for months.

The effect of temperature on fuel stability also complicates shipment of fuel as a liquid solution. To compensate for non-optimum shipping conditions and transit delays, the fuel should be stable under a variety of extreme conditions. Furthermore, transportation of large quantities of liquid fuel is impractical, as this would entail the movement of large amounts of water which adds to weight and cost.

Dry sodium borohydride does not suffer the same limitations as the solution and exhibits better stability since the reaction shown in Equation (1) does not occur without a water source. The solid borohydride could be shipped to the end usage point and be reconstituted into liquid fuel on site when needed. However, sodium borohydride is classed as a Division 4.3 “Dangerous When Wet” material by the U.S. Department of Transportation (DOT) in 49 CFR § 173.124(c), according to which a “Dangerous When Wet” material is a substance that upon reaction with water produces a dangerous amount of a gas which may be flammable. Appropriate packaging and labeling is required for these materials, and commercial shippers of hazardous materials (“HAZMAT”) are required to have specially trained employees to handle shipping activities of HAZMAT. This adds to the transportation cost.

For safety, convenience, and improved efficiencies, there is a need for solid fuel compositions that facilitate the transport and storage of a solid fuel, and enhance the use of borohydride salts as hydrogen storage and generation materials.

BRIEF SUMMARY OF THE INVENTION

In one embodiment, the present invention provides a solid fuel composition, comprising about 20 to about 99.7% by weight of a borohydride salt of formula M(BH₄)_(n), wherein M is selected from the group consisting of alkali metal cations, alkaline earth metal cations, aluminum cation, zinc cation, and ammonium cation, and n corresponds to the charge of the selected M cation; and a stabilizing amount of a hydroxide salt of formula M′(OH)_(n′), wherein M′ is selected from the group consisting of alkali metal cations, alkaline earth metal cations, aluminum cation, zinc cation, and ammonium cation, and n′ corresponds to the charge of the selected M′ cation, wherein the ratio by weight of borohydride salt to hydroxide salt is less than about 40:1.

A further embodiment of the invention provides a solid fuel composition, comprising of at least two borohydride salts of formula M(BH₄)_(n), wherein M is selected from the group consisting of alkali metal cations, alkaline earth metal cations, aluminum cation, zinc cation, and ammonium cation, and n corresponds to the charge of the selected M cation; and a stabilizing amount of at least one hydroxide salt of formula M′(OH)_(n′), wherein M′ is selected from the group consisting of alkali metal cations, alkaline earth metal cations, aluminum cation, zinc cation, and ammonium cation, and n′ corresponds to the charge of the selected M′ cation, wherein the combined weight of the borohydride salts is about 20 to about 99.7% by weight of the composition and the ratio by weight of the borohydride salts to hydroxide salts is less than about 40:1.

A further embodiment of the invention provides a stable fuel composition, comprising a borohydride salt of formula M(BH₄)_(n) and a hydroxide salt of formula M′(OH)_(n), wherein M and M′ are independently selected from the group consisting of alkali metal cations, alkaline earth metal cations, aluminum cation, zinc cation, and ammonium cation; wherein the composition is characterized by the production of gas at a rate less than about 1 liter per kilogram of material per hour averaged over a 7-hour period and is otherwise not considered a Division 4.3 “Dangerous When Wet” material according to the US Department of Transportation (DOT) in 49 CFR §173.124(c).

The invention also provides a stable fuel composition, comprising at least one solid form unit; the solid form unit containing a borohydride salt of formula M(BH₄)_(n) and a hydroxide salt of formula M′(OH)_(n), wherein M and M′ are independently selected from the group consisting of alkali metal cations, alkaline earth metal cations, aluminum cation, zinc cation, and ammonium cation; wherein the composition is characterized by the production of gas at a rate less than about 1 liter per kilogram of material per hour averaged over a 7-hour period and is otherwise not considered a Division 4.3 “Dangerous When Wet” material according to the US Department of Transportation (DOT) in 49 CFR §173.124(c).

In yet another embodiment the present invention provides a method of preparing a compacted solid fuel composition for hydrogen generation, the method comprising: providing a substantially uniform mixture of about 20 to about 99.7% by weight borohydride salt of formula M(BH₄)_(n) and about 0.3 to about 80% by weight hydroxide salt of formula M′(OH)_(n), wherein M and M′ are independently selected from the group consisting of alkali metal cations, alkaline earth metal cations, aluminum cation, zinc cation, and ammonium cation; and applying pressure to compact the mixture into at least one solid unit form having a density greater than the mixture.

The invention further provides a method of preparing a stable solid fuel composition for hydrogen generation, the method comprising: providing an aqueous solution comprising about 20 to about 99% by weight borohydride salt of formula M(BH₄)_(n) and about 0.3 to about 80% by weight hydroxide salt of formula M′(OH)_(n), wherein M and M′ are independently selected from the group consisting of alkali metal cations, alkaline earth metal cations, aluminum cation, zinc cation, and ammonium cation; and freeze drying the solution to produce a solid co-mingled mixture of borohydride and hydroxide salts substantially free of water.

The invention further provides a method of generating hydrogen gas, comprising: providing a solid fuel composition containing a homogeneous dispersion of about 20 to about 99.7% by weight borohydride salt of formula M(BH₄)_(n) and about 0.3 to about 80% by weight hydroxide salt of formula M′(OH)_(n), wherein M and M′ are independently selected from the group consisting of alkali metal cations, alkaline earth metal cations, aluminum cation, zinc cation, and ammonium cation; dissolving the solid fuel composition to prepare an aqueous fuel solution; and contacting the aqueous fuel solution with a catalyst to produce hydrogen gas.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention provides stable solid fuel compositions and methods for their preparation and hydrogen generation. Testing has shown that solid fuel compositions, comprising about 20 to about 99.7% by weight borohydride salt and a stabilizing amount of a hydroxide salt, wherein the ratio by weight of borohydride salt to hydroxide salt is less than about 40:1 according to the present invention are not subject to “dangerous when wet” classification. According to the US Department of Transportation (DOT) in 49 CFR §173.124(c) Division 4.3 “Dangerous When Wet” materials are defined as any substance that upon reaction with water produces a dangerous amount of gas which may be flammable, and specifically, is a substance that emits flammable gas at a rate greater than about 1 liter per kilogram of the substance per hour averaged over a 7-hour period. If the fuel composition according to the present invention becomes inadvertently wet during transport or storage, the molecular contacting of the hydroxide salt with the borohydride salt minimizes rapid hydrolysis and production of hydrogen gas.

By “stable” herein we mean that the solid fuel compositions according to the invention remain within about 99% of their initial strength (hydrogen gas production potential) at temperatures of below about 100° C. for at least about 36 months, more preferably, 60 months, and most preferable at least about 120 months. In another embodiment, the stability is defined as characterizing solid fuel compositions which need not be labeled or classified as dangerous-when-wet materials; in other words, materials that evolve gas at rates below those established for Division 4.3 “Dangerous When Wet” materials according to the U.S. Department of Transportation (DOT) in 49 CFR §173.124(c).

The solid compositions may be freeze dried mixtures made from solutions of at least one borohydride salt, with a cation selected from the group consisting of alkali metal cations, alkaline earth metal cations, aluminum cation, zinc cation, and ammonium cation, preferably sodium borohydride, and at least one hydroxide salt, with a cation selected from the group consisting of alkali metal cations, alkaline earth metal cations, aluminum cation, and ammonium cation.

The solid compositions may be compacted units made from mixtures of at least one borohydride salt, with a cation selected from the group consisting of alkali metal cations, alkaline earth metal cations, aluminum cation, zinc cation, and ammonium cation, preferably sodium borohydride, and at least one hydroxide salt, with a cation selected from the group consisting of alkali metal cations, alkaline earth metal cations, aluminum cation, zinc cation, and ammonium cation.

By “compacted” herein we mean that the fuel is in the form of a solid that substantially retains its shape during transport and storage, until subsequently disintegrated and/or dissolved to prepare an aqueous fuel solution. The solid forms are preferably made, as discussed in more detail below, by applying pressure to an intimate, homogenous mixture of salt components. The density of the solid forms can vary by application of different pressures and according to the needs and desires of the intended usage, but will typically be in the range of about, for example, 0.7 g/mL, preferably 0.8 g/mL, and more preferably about 0.9 g/mL to about 1.2 g/mL. Various other suitable densities may be selected by one of ordinary skill given the teachings herein.

In an alternative embodiment, the solid fuel is not compacted but is instead held together as a solid mass until use by any suitable means, including a binder, such as, for example, microcrystalline cellulose, spray processed lactose, anhydrous lactose, lactose monohydrate, dicalcium phosphate dihydrate, spray processed sucrose, or sucrose.

The borohydride fuel component of solid fuels according to the present invention is a complex metal hydride having the general formula M(BH₄)_(n) wherein M is a cation selected from the group consisting of alkali metal cations, such as sodium, potassium or lithium; alkaline earth metal cations, such as calcium aluminum cation; zinc cation; and ammonium cation, and n is equal to the charge of the cation. Preferred for such systems in accordance with the present invention are sodium borohydride (NaBH₄), lithium borohydride (LiBH₄), and potassium borohydride (KBH₄), including mixtures thereof. Sodium borohydride is generally preferred for hydrogen generation due to its gravimetric hydrogen storage density of 10.9%, its multi-million pound commercial availability, and its relative stability in alkaline aqueous solutions.

The stabilizer component of fuel compositions according to the invention is a hydroxide having the general formula M′(OH)n, wherein M′ is a cation, preferably selected from the group consisting of alkali metal cations, such as sodium, potassium or lithium; alkaline earth metal cations, such as calcium, and aluminum cation; and n is equal to the charge of the cation. Examples of suitable metal hydroxides, without limitation, include NaOH, LiOH, and the like. Other suitable metal hydroxides include NH₄OH and Zn(OH)₂. It is preferred that the cation portion of the alkaline stabilizing agent be the same as the cation of the metal hydride salt, i.e., that M′ is the same cation as M. If the metal borohydride is sodium borohydride, the preferred stabilizing agent would be sodium hydroxide, both of which are preferred in the practice of the present invention.

The mixtures show enhanced dissolution properties as compared to the commercially available borohydride-only caplets. Compressed sodium borohydride/sodium hydroxide caplets according to one embodiment of the present invention dissolved on average about 1 minute faster than like masses of the commercial sodium borohydride caplet in alkaline water, as shown in Table 1. Likewise, the non-compacted co-mingled mixtures of sodium borohydride and sodium hydroxide dissolved faster than a similar mass of sodium borohydride only. TABLE 1 Physical Properties Average Dissolution Dimensions Density Mass per surface Times (minutes) (mm) (g/mL) area (g/cm²) 3.1° C. 21° C. 46° C. SBH granules (a) 1.074 0.045 1.6 1.0 0.5 Comingled SBH granules and SH (a) — 0.045 0.65 0.5 0.16 Compacted SBH and SH puck 50 × 10 0.929 0.31 4.8 3.6 1.8 Compacted SBH caplets 18 × 10 × 8 1.074 0.25 6.2 4.4 2.0 SBH = sodium borohydride; SH = sodium hydroxide; (a) individual SBH granules have typical individual particle sizes of 1 to 3 mm. Approximately 17 g of solid materials were dissolved in approximately 70 g of solvent per test such that the resulting solution had a concentration of 20% by weight sodium borohydride and 3% by weight sodium hydroxide

Aqueous fuel solutions may be prepared by combining various optional components, a solvent, typically water, and a solid fuel according to the invention. The water component may contain other additives, for example, anti-freeze agents such as ethylene glycol. The solid borohydride fuel may be prepared such that the borohydride and the hydroxide components are present in a pre-determined ratio to allow preparation of specific fuel solution concentrations. Fuel packages or cartridges comprising a housing containing a solid fuel mixture represent significant improvements in storage and transportation. The liquid fuel solution can be readily prepared in the fuel package by the addition of water and agitation of the resulting solution. Suitable fuel cartridges include but are not limited to those disclosed in copending application Ser. No. 10/359,104, filed Feb. 5, 2003, incorporated by reference herein in its entirety. The improved dissolution properties ensure that the fuel solution is prepared rapidly. Representative examples given in Table 2 illustrate fuel mixtures that can produce fuel solutions of about 5 to 35 wt-% sodium borohydride and about 0.1 to about 40 wt-% sodium hydroxide. These mixtures can be alternatively expressed in terms of the ratio between the borohydride and hydroxide salts rather than as percentage of total weight; the fuel formulations shown below include hydroxide salt in an amount of from about 0.28 to about 800% of the borohydride salt on a weight basis. TABLE 2 Component Solid Fuel Solution, Weight (g) Mixture, Wt-%, Wt-% (a) Formulation NaBH₄ NaOH NaBH₄ NaOH NaBH₄ NaOH A 7.83 0.078 99 1 10 0.1 B 27.4 0.078 99 0.3 35 0.1 C 27.4 3.92 87.5 12.5 35 5 D 15.66 2.34 87 13 20 3 E 7.83 3.92 67 33 10 5 F 9.40 31.32 23.1 76.9 12 40 (a) for 78.3 g fuel solution, e.g., sufficient water added to solid mixture to produce 78.3 g total solution

The borohydride component of the fuel formulations shown in the previous table is anhydrous (see, e.g., U.S. Pat. Nos. 6,231,825 and 2,542,746, hereby incorporated herein by reference in their entireties). However, the hydrated forms of certain borohydride salts, notably sodium borohydride, are common at low to moderate temperatures. For example, sodium borohydride dihydrate is formed at temperatures below 36.4° C., potassium borohydride trihydrate exists at temperatures below 7.5° C., and potassium borohydride monohydrate exists at temperatures below 37.5° C.; sodium borohydride dihydrate, NaBH₄.2H₂O, is a solid containing 51.2 wt-% sodium borohydride and 48.8 wt-% water. As some of the necessary water for hydrolysis is thus incorporated into the solid mixture, the hydrates also are useful components of solid fuel compositions according to this invention.

Such hydrates can be produced in solid compositions according to the present invention by, for example, adding water to the compounding mixture. Representative examples are provided in Table 3 for sodium borohydride hydrates. In such cases, water may be present in any suitable amount. Water may be present in the solid mixture at, for example, between about 44 and 90% of the borohydride salt on a weight basis, and hydroxide may be present at, for example, between about 1 and 50% of the borohydride salt on a weight basis. TABLE 3 Component Weight (g) Compacted Solid, Wt-%, Solution, Wt-% Formulation NaBH₄ NaOH H₂O NaBH₄ NaOH H₂O NaBH₄ NaOH H₂O A 8 0.08 0 99.0 1.0 0.0 10.0 0.1 89.9 B 27.5 0.08 0 99.7 0.3 0.0 34.4 0.1 65.5 C 27.5 4 0 87.3 12.7 0.0 34.4 5.0 60.6 D 16 2.4 0 87.0 13.0 0.0 20.0 3.0 77.0 E 7.8 3.92 0 66.6 33.4 0.0 9.8 4.9 85.4 F 9.4 31.3 0 23.1 76.9 0.0 11.8 39.1 49.1 G 7.8 0.08 4.67 62.2 0.6 37.2 9.8 0.1 96.0 H 7.8 3.92 7.4 40.8 20.5 38.7 9.8 4.9 94.6 I 9.4 2.4 6.58 51.1 13.1 35.8 11.8 3.0 93.5 J 9.4 0.094 7.52 55.2 0.6 44.2 11.8 0.1 97.5

Compacted compositions of borohydride and hydroxide salts produced in a discrete form ensure that each individual unit contains a specified ratio of hydroxide stabilizer and borohydride to provide accurate concentrations for use as a fuel solution. Such compositions also exhibit improved bulk density properties as compared to powder mixtures; bulk mixtures of different components can be prone to segregation on storage either in a drum as might be used for transport, or in a hopper as might be encountered in some dispensing systems. Such segregation can result in a heterogeneous mixture in which the stabilizer (e.g., the hydroxide salt) is no longer evenly dispersed within the active fuel (e.g., borohydride salt) and make it difficult to ensure accurate concentrations of both components in the fuel from batch to batch. As compressed units will not compact together, they enable consistent dispensing. Thus, the compacted forms are particularly suited for those applications requiring automated and repeated dispensing from a bulk storage system to produce a fuel solution.

The compacted fuel compositions can be compressed into a variety of solid units including, but not limited to, caplets, tablets, and granules. Tablets are flattened cylinders measured in two dimensions with a diameter greater than the height, caplets are elongated tablets measured in three dimensions (length, width, and height), and granules are fragments of a larger unit that are typically sieve-sorted according to grain size. Other solid shapes, including, but not limited to, spheres, rectangular parallelepipeds and cubes, are also contemplated by the present invention and can be chosen in accordance with various dispensing mechanisms. The choice of shape and size depends on the scale of the hydrogen generator and is readily determined given the teachings herein by one skilled in the art and familiar with the demands of a particular system, e.g., dissolution rate, relative composition of hydroxide in the mixture, and humidity tolerance. Fuel compositions with a high concentration of hydroxide salt can absorb water readily from the air. In high humidity environments, such compositions are best used shortly after preparation or, if they must be stored, protected from the environment such as under an inert gas atmosphere, Alternatively, the composition can be coated with a water soluble material such as, for example, a gelatin.

The surfaces of the cylinder shapes may be smooth or textured, for example, with convex scallop cut outs, which can serve to prevent individual units from sticking together. In addition, the solid compacted compositions can be scored or grooved, which can serve to cause the units to break apart when mixed and to aid dissolution. The scoreline can be used to divide a larger compacted unit into smaller portions so that either several smaller batches or one larger batch could be prepared from one individual unit. The density of compaction can also affect dissolution in that more dense compactions will tend to dissolve at a slower rate. Any combination of these features may be present in a given formulation to fulfill the needs of the hydrogen generator system or end use.

Such compacted fuel components can be utilized in a variety of batch and indexed fueling systems. In this context, a batch system may use a metering device that delivers multiple tablets to a mixing tank to prepare a batch of fuel for delivery to a hydrogen generation system, while an indexed system may deliver one individual tablet at a time. For instance, an indexed system configured to make 78.3 g batches of 20 wt-% NaBH₄ and 3 wt-% NaOH fuel could use one 18 g tablet of Formulation D from Table 2 above. A batch system designed to prepare the same 78.3 g batch of fuel could use eighteen 1 g tablets containing 0.87 g NaBH₄ and 0.13 g NaOH. In both instances, a total of 15.66 g NaBH₄ and 2.34 g NaOH would be delivered. Suitable automated mixing systems are described in U.S. patent application Ser. No. 10/115,269 entitled “Method And System For Generating Hydrogen By Dispensing Solid and Liquid Fuel Components,” filed Apr. 2, 2002, the disclosure of which is herein incorporated by reference in its entirety, wherein the fuel for the hydrolysis reaction is generated on an “as needed” basis using solid and liquid fuel components.

Optional additives also can be added to the compacted fuel formulations, including additives to promote disintegration or dissolution. For example, compounds which effervesce when added to water can be added so that the action of the gas bubbles helps break up the tablet and provides mixing action to help dissolve the borohydride salt. Suitable effervescent systems include, for example, mixtures of acids, such as citric acid, tartaric acid, fumaric acid, adipic acid, and maleic acid, with carbonate or bicarbonate salts, such as sodium carbonate, sodium bicarbonate, potassium carbonate, and magnesium carbonate. Acids such as maleic acid, citric acid, and tartaric acid will accelerate hydrolysis and hydrogen generation when combined with sodium borohydride in amounts greater than about 50% of the borohydride salt on a weight basis; in order to use an acid/carbonate mixture for effervescence, the acids are preferably present in a lower concentration, such as about 10% or less of the borohydride salt on a weight basis. A specific non-limiting example of an effervescent fuel tablet comprises 15.66 g sodium borohydride, 2.34 g sodium hydroxide, 1 g citric acid, and 1.7 g sodium bicarbonate. The carbonate/acid mixture may be homogeneously distributed throughout the compacted fuel composition or, alternatively, formed into a small pellet, which is then incorporated into the compacted fuel.

Co-pending U.S. patent application Ser. No. 10/741,199, entitled “Fuel Blends for Hydrogen Generators,” filed Dec. 19, 2003, incorporated by reference herein in its entirety, describes the use of certain acidic salts of polyhedral boron hydrides such as diammonium decahydrodecaborate, (NH₄)₂B₁₀H₁₀, as accelerants to promote the hydrolysis of sodium borohydride. In the presence of water, it is thought that (NH₄)₂B₁₀H₁₀ and NaBH₄ react to form (NH₄)BH₄, which hydrolyzes rapidly. As accelerants, the acidic salts are preferably present in amounts greater than about 0.5% of sodium borohydride on a by weight basis, e.g., greater than about 25 mg (NH₄)₂B₁₀H₁₀ for 5 g of NaBH₄. In amounts less than about 0.5% of sodium borohydride, the polyhedral boron hydride is less able to effectively accelerate the hydrolysis reaction to completion. However, at these lower concentrations, the vigorous reaction of (NH₄)BH₄ which does form also can act as a disintegrant and aid in breaking apart the compacted mass and thus speeding dissolution.

The solid borohydride fuel compositions provided herein are suitable for the preparation of aqueous solutions for the generation of hydrogen as described in U.S. Pat. No. 6,534,033, entitled “A System for Hydrogen Generation,” the content of which is hereby incorporated herein by reference in its entirety. A catalyst is typically needed for effective hydrogen generation and suitable transition metal catalysts for the generation of hydrogen from a metal hydride solution include metals from Group 1B to Group VIIIB of the Periodic Table, either utilized individually or in mixtures, or as compounds of these metals. Representative examples of these metals include, without intended limitation, transition metals represented by the copper group, zinc group, scandium group, titanium group, vanadium group, chromium group, manganese group, iron group, cobalt group and nickel group. Specific examples of useful catalyst metals include, without intended limitation, ruthenium, iron, cobalt, nickel, copper, manganese, rhodium, rhenium, platinum, palladium, and chromium. The catalyst may also take the form of beads, rings, pellets or chips, and involve structured catalyst supports such as honeycomb monoliths or metal foams. The preparation of supported catalysts is taught, for example, in U.S. Pat. No. 6,534,033.

The following examples further describe and demonstrate features and methods for the preparation of compacted fuel compositions according to the present invention. The examples are given solely for illustration purposes and are not to be construed as a limitation of the present invention. Various other compositions and methods of manufacture can be used and will be readily ascertainable to one skilled in the art given the teachings herein.

EXAMPLE 1

A fuel tablet containing 87 wt-% sodium borohydride and 13 wt-% sodium hydroxide was prepared by mixing 15.2 grams of NaBH₄ and 2.28 grams of NaOH using a mortar and pestle. The powder mixture was placed in a housing and compressed in a hydraulic press using 10,000 lbs of force to produce a puck of 2 inches in diameter. The degree of compaction and the height of the puck were controlled by the amount of force used in compression. When dissolved in water, the tablet produced fuel solutions with sodium borohydride and sodium hydroxide in a 6.7 to 1 ratio by weight, as illustrated in Table 4 below. TABLE 4 Fuel Wt-% In Fuel Solution Water, g Solution, g NaBH₄ NaOH 35.52 50 30.4 4.56 58.52 76 20 3 85.52 100 15.2 2.28 182.52 200 7.6 1.14

EXAMPLE 2

Solid hydrated sodium borohydride fuel mixtures were created by combining water, anhydrous NaBH₄ powder, and anhydrous NaOH in the proportions shown in Table 5 below; (for reference, sodium borohydride hydrate, NaBH₄.2H₂O, is a solid containing 51.2 wt-% sodium borohydride and 48.8 wt-% water): TABLE 5 Composition wt-% NaBH₄ wt-% NaOH wt-% H₂O 1 55 0.1 44.9 2 60 0.1 39.9 3 70 0.1 29.9 4 80 0.1 19.9 5 90 0.1 9.9 (a) Pure solid sodium borohydride dehydrate

These solids became progressively more opaque and more “monolithic” as the proportion of sodium borohydride increased. That is, the fuel mixture of Composition 1 had better powder flow characteristics and could be shaken out of its container more easily than the fuel mixture of Composition 5. The higher concentrations become progressively more water deficient.

The samples as prepared above were dissolved in sufficient water to obtain fuel solutions of approximately 20 wt-% NaBH₄ (for example, 100 g of sodium borohydride dihydrate dissolved in 156 mL of water). The resulting fuel solutions generate hydrogen when exposed to a hydrogen generation catalyst.

EXAMPLE 3

Fuel compositions of the present invention were tested according to established test methods for substances which, on contact with water, emit flammable gases. A fuel mixture was prepared by combining 320 g of granular sodium borohydride and 48 g of granular sodium hydroxide in a flask and shaking the flask to mix. The fuel composition was tested according to the following procedures:

(1) A quantity of the fuel mixture measuring approximately 2 mm in diameter was placed in a trough of distilled water at 20° C. Gas evolution was noted, but no spontaneous ignition of the gas occurred.

(2) A quantity of the fuel mixture measuring approximately 2 mm diameter was placed on the center of a filter paper which was floated flat on the surface of distilled water at 20° C. in a 100 mm diameter evaporating dish. Gas evolution was noted, but no spontaneous ignition of the gas occurred.

(3) A pile approximately 20 mm high and 30 mm diameter with a hollow in the top was made from the fuel mixture, and a few drops of water were added to the hollow. Gas evolution was noted, but no spontaneous ignition of the gas occurred.

(4) The rate of gas evolution was determined at ambient temperature (about 20° C.) and atmospheric pressure by placing a sample of the fuel composition into a conical flask, and adding water from a dropping funnel. The volume of gas evolved was measured by volume displacement, and the rate of evolution calculated at 1 hour intervals over 7 hours. The results for three samples are summarized in Table 6. TABLE 6 Test No. 1 Test No. 2 Test No. 3 Time (2.0 g sample) (10.0 g sample) (25.0 g sample) (hours) Displacement (mL) Displacement (mL) Displacement (mL) 1 0 0 12 2 0 0 0 3 0 0 0 4 0 0 0 5 0 0 0 6 0 0 0 7 0 0 0 Total 0 0 12 Rate of 0.0 L/kg/hr 0.0 L/kg/hr 0.5 L/kg/hr Gas Evo- lution

EXAMPLE 4

A solution containing 20 wt-% sodium borohydride and 3 wt-% sodium hydroxide was lyophilized by a 13-step process involving freezing the solution and heating the frozen mixture under vacuum to remove solvent. This process is summarized in Table 7. TABLE 7 Segment Time (hours) Shelf Temperature (° C.) Vacuum (microns)  1 10 Ramp −50 to −14 <100  2 5 −14 <100  3 4 −14 120  4 5 −14 150  5 9 −14 200  6 5 −5 250  7 2 0 250  8 1.5 5 250  9 5 5 275 10 13 10 275 11 7 20 275 12 13 25 300 13 13 30 350 Total 92.5 — —

Physical analyses of this solid showed that it was unique from a physically mixed non-lyophilized composition containing the same proportion of sodium hydroxide and sodium borohydride components. Powder X-ray diffraction of the freeze dried material showed strong peaks at 25°, 29°, 41.5°, 49° and 51.5° indicative of sodium borohydride in contrast to the physically mixed version which contained these peaks and those representative of sodium hydroxide and sodium hydroxide hydrate (major peaks: 15°, 16°, 30.5°, 31.5°, 33.5°, 36.5°, 38.5°, 54°, and 56°). Optical microscopy under polarizing light indicated that a homogeneous mixture of borohydride and hydroxide salts is formed during lyophilization while non-lyophilized mixtures leaves segregated particles. The material produced from lyophilization could be readily dissolved in water to form a solution that generates hydrogen when exposed to a hydrogen generation catalyst. The conversion of the borohydride ion to borate ion was confirmed by NMR.

EXAMPLE 5

Experiments were performed to compare dissolution rates of granules and compacted caplets containing only sodium borohydride (Table 8a) and compacted pucks containing a mixture of sodium borohydride and sodium hydroxide (Table 8b). The granules and caplets are commercially available, for example, from Rohm and Haas Company (Philadelphia, Pa.). The pucks were prepared by compacting a mixture of sodium borohydride and sodium hydroxide in a 20:3 ratio by weight to produce a circular disc, 50 mm in diameter and 10 mm thick, with a density of approximately 0.929 g/cc.

The sodium borohydride only samples were added to a beaker of stirring water containing dissolved NaOH such that the resulting fuel solutions had a concentration of 20% by weight sodium borohydride and 3% by weight sodium hydroxide. The compacted pucks were added to stirring water to produce a fuel solution with a concentration of 20% by weight sodium borohydride and 3% by weight sodium hydroxide. The solutions were stirred using a magnetic stir bar. TABLE 8a Sodium Borohydride Caplets Sample # Sample Wt. (g) Solvent (g) Mix Time (min) 1 17.0 68.0 4.5 2 17.0 68.0 4.0 3 17.2 68.8 4.0 4 17.1 68.4 4.5 5 17.1 68.4 4.5 6 17.0 68.0 4.5 7 17.0 68.0 4.5 8 17.0 68.0 4.5 9 17.0 68.0 4.5 10 17.0 68.0 4.5 Avg. Rate of Dissolution 0.06 g/mL/sec

TABLE 8b Sodium Borohydride/Sodium Hydroxide Caplets Sample # Sample Wt. (g) Solvent (g) Mix Time (min) 1 17.6 70.4 3.0 2 17.7 70.8 3.0 3 17.6 70.4 3.5 4 17.7 70.8 3.0 5 17.6 70.4 3.0 Avg. Rate of Dissolution 0.07 g/mL/sec

While the present invention has been described with respect to particular disclosed embodiments, it should be understood that numerous other embodiments are within the scope of the present invention. For instance, while the examples given show only sodium borohydride, the compacted fuel compositions may include other compounds or mixtures of borohydride compounds, in order to customize gravimetric hydrogen storage density and hydrogen generation rate to particular end use applications. The compacted compositions may also include one or a combination of various optional features to aid in dissolution. For example, an effervescent tablet may also include scoring to accelerate the disintegration of the compacted mass.

Although the invention has been described in detail in connection with the exemplary embodiments, it should be understood that the invention is not limited to the above disclosed embodiments. Rather, the invention can be modified to incorporate any number of variations, alterations, substitutions, or equivalent arrangements not heretofore described, but which are commensurate with the spirit and scope of the invention. Accordingly, the invention is not limited by the foregoing description, but is only limited by the scope of the appended claims. 

1. A solid fuel composition, comprising: about 20 to about 99.7% by weight borohydride salt of formula M(BH₄)_(n), wherein M is selected from the group consisting of alkali metal cations, alkaline earth metal cations, aluminum cation, zinc cation, and ammonium cation, and n corresponds to the charge of the selected M cation; and a stabilizing amount of a hydroxide salt of formula M′(OH)_(n′), wherein M′ is selected from the group consisting of alkali metal cations, alkaline earth metal cations, aluminum cation, zinc cation, and ammonium cation, and n′ corresponds to the charge of the selected M′ cation.
 2. The composition of claim 1, wherein the borohydride and hydroxide salts are homogeneously distributed within the solid fuel composition.
 3. The composition of claim 1, wherein the hydroxide salt is present in an amount of about 0.3 to about 80% by weight.
 4. The composition of claim 1, wherein M and M′ are the same cation.
 5. The composition of claim 1, wherein the borohydride salt is selected from the group consisting of sodium borohydride, lithium borohydride, potassium borohydride, calcium borohydride, and mixtures thereof.
 6. The composition of claim 1, wherein the borohydride salt is hydrated.
 7. The composition of claim 6, wherein water is present in the solid fuel in an amount of about 1 to about 96% by weight of the borohydride salt.
 8. The composition of claim 6, wherein the borohydride salt is selected from the group consisting of sodium borohydride dihydrate, potassium borohydride trihydrate, potassium borohydride monohydrate, and mixtures thereof.
 9. The composition of claim 1, wherein the hydroxide salt is selected from the group consisting of calcium hydroxide, sodium hydroxide, lithium hydroxide, potassium hydroxide, and mixtures thereof.
 10. The composition of claim 9, wherein the hydroxide salt is present in an amount of about 0.28 to about 800% by weight of the borohydride salt.
 11. The composition of claim 1, comprising a compacted solid fuel.
 12. The composition of claim 11, wherein the compacted solid fuel is in the shape of a tablet.
 13. The composition of claim 11, wherein the compacted solid fuel is in the shape of a caplet.
 14. The composition of claim 11, wherein the compacted solid fuel is in the shape of a granule.
 15. The composition of claim 11, wherein the compacted solid fuel has a shape selected from the group consisting of rectangular parallelepipeds and spheres.
 16. The composition of claim 11, wherein at least one surface of the compacted solid fuel is textured.
 17. The composition of claim 11, wherein at least one surface of the compacted solid fuel is scored.
 18. The composition of claim 11, further comprising a solid acid component and a solid carbonate component for producing carbon dioxide.
 19. The composition of claim 18, wherein the solid acid component is present in an amount of about 1 to about 10% by weight of the borohydride salt.
 20. The composition of claim 18, wherein the solid acid component is selected from the group consisting of citric acid, tartaric acid, fumaric acid, adipic acid, maleic acid, and oxalic acid.
 21. The composition of claim 18, wherein the solid carbonate component is selected from the group consisting of sodium carbonate, sodium bicarbonate, potassium carbonate, and magnesium carbonate.
 22. The composition of claim 18, wherein at least one of the solid acid component and the solid carbonate component is homogenously distributed within the solid fuel.
 23. The composition of claim 18, wherein at least one of the solid acid component and the solid carbonate component is present in a compressed pellet inside the solid fuel.
 24. The composition of claim 11, further comprising a disintegrant.
 25. The composition of claim 24, wherein the disintegrant is diammonium decahydrodecaborate.
 26. The composition of claim 25, when the disintegrant is present in an amount from about 0.1 to about 0.5% by weight of the borohydride salt.
 27. A stable fuel composition, comprising a mixture of a borohydride salt of formula M(BH₄)_(n) and a hydroxide salt of formula M′(OH)_(n), wherein M and M′ are independently selected from the group consisting of alkali metal cations, alkaline earth metal cations, aluminum cation, zinc cation, and ammonium cation, wherein the composition upon reaction with water is characterized by the production of gas at a rate less than about 1 liter per kilogram of material per hour averaged over about a 7-hour period.
 28. The fuel composition of claim 27, wherein the borohydride salt is present in an amount of about 20 to about 99.7% by weight of the solid fuel composition.
 29. The fuel composition of claim 27, wherein the hydroxide salt is present in an amount of about 0.3 to about 80% by weight of the solid fuel composition.
 30. The fuel composition of claim 27, wherein the composition dissolves in aqueous solution at room temperature at a rate of about 0.05 g/mL/sec to about 0.5 g/mL/sec.
 31. The fuel composition of claim 27, wherein the fuel composition comprises a solid form unit selected from the group consisting of a caplet, a tablet, a granule, a parallelepiped, a sphere, and a cube.
 32. The fuel composition of claim 31, wherein at least one surface of the solid form unit is textured or scored.
 33. The fuel composition of claim 31, wherein the solid form unit has a density of about 0.7 g/mL to about 1.2 g/mL.
 34. The fuel composition of claim 31, wherein the solid form unit further comprises an effervescent component.
 35. The fuel composition of claim 31, wherein the solid form unit further comprises a disintegrating component.
 36. The fuel composition of claim 31, wherein the solid form unit further comprises a binder.
 37. The fuel composition of claim 27, wherein water is present in the composition in an amount of about 1 to about 96% by weight of the borohydride salt.
 38. The fuel composition of claim 37, wherein the borohydride salt is selected from the group consisting of sodium borohydride dehydrate, potassium borohydride trihydrate, potassium borohydride monohydrate, and mixtures thereof.
 39. The fuel composition of claim 37, wherein the hydroxide salt is selected from the group consisting of sodium hydroxide, lithium hydroxide, potassium hydroxide, and mixtures thereof.
 40. A method of preparing a compacted solid fuel composition for hydrogen generation, the method comprising: providing a uniform mixture of about 20 to about 99.7% by weight borohydride salt of formula M(BH₄)_(n) and about 0.3 to about 80% by weight hydroxide salt of formula M′(OH)_(n), wherein M and M′ are independently selected from the group consisting of alkali metal cations, alkaline earth metal cations, aluminum cation and ammonium cation; and applying pressure to compact the mixture into at least one solid unit form having a density greater than the mixture.
 41. The method of claim 40, further comprising compacting the mixture in a mold to produce at least one solid unit form selected from the group consisting of a caplet, a tablet, a granule, a sphere, a parallelepiped, and a cube.
 42. The method of claim 40, wherein the borohydride salt is selected from the group consisting of sodium borohydride, lithium borohydride, potassium borohydride, calcium borohydride, and mixtures thereof.
 43. The method of claim 40, further comprising adding water to the mixture in an amount of about 1 to about 96% by weight of the borohydride salt.
 44. The method of claim 40, wherein the borohydride salt is a hydrated borohydride salt selected from the group consisting of sodium borohydride dihydrate, potassium borohydride trihydrate, potassium borohydride monohydrate, and mixtures thereof.
 45. The method of claim 40, wherein the hydroxide salt is selected from the group consisting of sodium hydroxide, lithium hydroxide, potassium hydroxide, and mixtures thereof.
 46. The method of claim 40, further comprising admixing a solid acid component and a solid carbonate component for producing carbon dioxide.
 47. The method of claim 46, wherein the solid acid component is present in an amount of about 1 to about 10% by weight of the borohydride salt.
 48. The method of claim 45, wherein the solid acid component is selected from the group consisting of citric acid, tartaric acid, fumaric acid, adipic acid, maleic acid, and oxalic acid.
 49. The method of claim 45, wherein the solid carbonate component is selected from the group consisting of sodium carbonate, sodium bicarbonate, potassium carbonate, and magnesium carbonate.
 50. The method of claim 45, further comprising homogenously distributing the solid acid component and the solid carbonate component within the mixture.
 51. The method of claim 40, further comprising admixing at least one of the solid acid component and the solid carbonate component as a compressed pellet in the mixture.
 52. The composition of claim 34, further comprising admixing a disintegrant with the mixture.
 53. The composition of claim 52, wherein the disintegrant is diammonium decahydrodecaborate.
 54. The composition of claim 52, when the disintegrant is present in an amount from about 0.1 to about 0.5% by weight of the borohydride salt.
 55. A method of generating hydrogen gas, comprising: providing a solid fuel composition containing a homogeneous dispersion of about 20 to about 99.7% by weight borohydride salt of formula M(BH₄)_(n) and about 0.3 to about 80% by weight hydroxide salt of formula M′(OH)_(n), wherein M and M′ are independently selected from the group consisting of alkali metal cations, alkaline earth metal cations, aluminum cation, zinc cation, and ammonium cation; dissolving the solid fuel composition to prepare an aqueous fuel solution; and contacting the aqueous fuel solution with a catalyst to produce hydrogen gas.
 56. The method of claim 55, wherein the solid fuel composition further comprises a solid acid component and a solid carbonate component for producing carbon dioxide.
 57. The method of claim 56, wherein the solid acid component is present in an amount of about 1 to about 10% by weight of the borohydride salt.
 58. The method of claim 56, wherein the solid acid component is selected from the group consisting of citric acid, tartaric acid, fumaric acid, adipic acid, maleic acid, and oxalic acid.
 59. The method of claim 56, wherein the solid carbonate component is selected from the group consisting of sodium carbonate, sodium bicarbonate, potassium carbonate, and magnesium carbonate.
 60. The method of claim 55, wherein the solid fuel composition further comprises a disintegrant.
 61. The method of claim 60, wherein the disintegrant is diammonium decahydrodecaborate.
 62. The method of claim 60, when the disintegrant is present in an amount from about 0.1 to about 0.5% by weight of the borohydride salt.
 63. The method of claim 54, wherein the catalyst is selected from the group consisting of metals of ruthenium, iron, cobalt, nickel, copper, manganese, rhodium, rhenium, platinum, palladium, and chromium, and salts of ruthenium, iron, cobalt, nickel, copper, manganese, rhodium, rhenium, platinum, palladium, and chromium.
 64. A compacted solid fuel composition for hydrogen generation, comprising about 20 to about 99.7% by weight sodium borohydride salt; about 0.3 to about 80% by weight sodium hydroxide salt homogeneously interdispered with the sodium borohydride salt; and wherein the compacted solid fuel composition is in a form selected from the group consisting of caplets, tablets, granules, spheres, parallelepipeds and cubes, and has a density of from about 0.7 to about 1.2 g/mL.
 65. The composition according to claim 64, further comprising a solid acid component and a solid carbonate component.
 66. The solid fuel composition of claim 64, wherein water is present in the solid fuel in an amount of about 1 to about 96% by weight of the borohydride salt.
 67. The solid fuel compositions of claim 64, wherein the borohydride salt is sodium borohydride dihydrate.
 68. The solid fuel composition of claim 64, wherein the hydroxide salt is present in an amount of about 1% to 50% of the borohydride salt.
 69. The solid fuel composition of claim 64, wherein the sodium borohydride is present in about 87 weight percent and the sodium hydroxide is present in about 13 weight percent.
 70. The solid fuel composition of claim 64, wherein the solid fuel further comprises an effervescent component.
 71. The solid fuel composition of claim 64, wherein the solid fuel further comprises a disintegrating component.
 72. The solid fuel composition of claim 64, wherein the solid form is scored or textured.
 73. The solid fuel composition of claim 64, wherein the composition is not subject to dangerous-when-wet classification and is characterized by the production of gas at a rate less than about 1 liter per kilogram of material per hour averaged over about a 7-hour period.
 74. A method of preparing a solid fuel composition for hydrogen generation, the method comprising: providing a solution of about 2 to about 35% by weight borohydride salt of formula M(BH₄)_(n) and about 0.5 to about 40% by weight hydroxide salt of formula M′(OH)_(n), in water, wherein M and M′ are independently selected from the group consisting of alkali metal cations, alkaline earth metal cations, aluminum cation and ammonium cation; cooling said solution to produce a frozen solution; and subliming the water from said frozen solution to produce a solid mixture substantially free of solvent.
 75. A solid fuel composition, comprising: about 20 to about 99.7% by weight borohydride salt of formula M(BH₄)_(n), wherein M is selected from the group consisting of alkali metal cations, alkaline earth metal cations, aluminum cation, zinc cation, and ammonium cation, and n corresponds to the charge of the selected M cation; and a stabilizing amount of a hydroxide salt of formula M′(OH)_(n′), wherein M′ is selected from the group consisting of alkali metal cations, alkaline earth metal cations, aluminum cation, zinc cation, and ammonium cation, and n′ corresponds to the charge of the selected M′ cation, wherein the solid fuel composition has an X-Ray powder diffraction (XRPD) pattern with peaks at 25, 29, 41.5, 49 and 51.5 degrees 2-theta.
 76. A solid fuel composition, comprising: about 20 to about 99.7% by weight borohydride salt of formula M(BH₄)_(n), wherein M is selected from the group consisting of alkali metal cations, alkaline earth metal cations, aluminum cation, zinc cation, and ammonium cation, and n corresponds to the charge of the selected M cation; and a stabilizing amount of a hydroxide salt of formula M′(OH)_(n′), wherein M′ is selected from the group consisting of alkali metal cations, alkaline earth metal cations, aluminum cation, zinc cation, and ammonium cation, and n′ corresponds to the charge of the selected M′ cation, and wherein the ratio by weight of the borohydride salt to the hydroxide salt is less than about 40:1.
 77. A solid fuel composition, comprising: at least two salts of formula M(BH₄)_(n) and M′(BH₄)_(n′), wherein M and M′ are independently selected from the group consisting of alkali metal cations, alkaline earth metal cations, aluminum cation, zinc cation, and ammonium cation, and n and n′ correspond to the charge of the selected M and M′ cation, respectively; and a stabilizing amount of a hydroxide salt of formula M″(OH)_(n″), wherein M″ is selected from the group consisting of alkali metal cations, alkaline earth metal cations, aluminum cation, zinc cation, and ammonium cation, and n″ corresponds to the charge of the selected M″ cation, and wherein the solid fuel composition upon reaction with water produces gas at a rate less than about 1 liter per kilogram of material per hour averaged over about a 7-hour period.
 78. A method of preparing a solid fuel composition for hydrogen generation, the method comprising: providing about 20 to about 99.7% by weight borohydride salt of formula M(BH₄)_(n), wherein M is selected from the group consisting of alkali metal cations, alkaline earth metal cations, aluminum cation, zinc cation, and ammonium cation, and n corresponds to the charge of the selected M cation; providing a stabilizing amount of a hydroxide salt of formula M′(OH)_(n′), wherein M′ is selected from the group consisting of alkali metal cations, alkaline earth metal cations, aluminum cation, zinc cation, and ammonium cation, and n′ corresponds to the charge of the selected M′ cation, wherein the ratio by weight of the borohydride salt to the hydroxide salt is less than about 40:1l; and mixing the borohydride salt and the hydroxide salt to form a substantially uniform mixture.
 79. The method of claim 55, wherein the solid fuel composition dissolves at a rate of about 0.05 g/mL/sec to about 0.5 g/mL/sec.
 80. The method of claim 55, wherein the solid fuel composition has a density of about 0.7 to about 1.2 g/mL.
 81. The method of claim 55, wherein the solid fuel composition has an x-ray powder diffraction (XRPD) pattern with peaks at 25, 29, 41.5, 49 and 51.5 degrees 2-theta.
 82. A fuel cartridge, comprising: a housing for containing a solid fuel; and a solid fuel composition according to claim 1 contained within the housing.
 83. The composition of claim 1, wherein the hydroxide salt is selected from the group consisting of alkaline earth metal salts and alkaline metal salts. 